It is well known to provide propulsion units that are suspended below the hull of a ship in order to provide the ship with propulsion. Examples of such propulsion units are shown in U.S. Pat. Nos. 6,231,407 and 5,101,128.
The unit shown in U.S. Pat. No. 6,231,407 contains an electric motor having a permanent magnet rotor. Such motors are attractive because they suffer from low losses in the rotor and therefore do not generate excessive heat. Such heat generated by the rotor can relatively easily be dissipated by conduction/convection to the remainder of the unit. However, such motors are disadvantageous because the permanent magnets used to generate the rotor magnetic field are expensive, it is hard to assemble the rotor, and a suitable power converter system must be employed. The power converter system must be able to cope with large changes in power factor with load, which such a motor produces because there is no control of the rotor excitation.
U.S. Pat. No. 5,101,128 shows a further example of a propulsion unit using water cooling, which incorporates a bellows and filter to equalise water pressure, from outside of the unit. The arrangement provided is complex, which leads to increased manufacturing and maintenance costs. Increased maintenance is a particular disadvantage due to the poor access to the units, and therefore, it is generally desired to make the units as simple as possible.
It is an aim of the present invention to overcome, or at least reduce, the problems of the prior art.